Catalytic surfaces are provided on polymer substrates for a variety of purposes, including to provide initiation for electroless deposition of metal. Electroless deposition as used herein refers to the surface deposition from solution of a reduced metal coating onto a substrate by use of a chemical reducing agent without an outside source of electric current. Electroless deposition is used to deposit such metals as nickel, copper, gold, rhodium and palladium onto non-conductive substrates, e.g. polymeric substrates, to provide printed circuit boards, laser or magnetic data storage devices, catalytic devices, electromagnetic shielding of electronic equipment housings, conductive coatings, decorative coatings, antistatic coatings and the like.
Electroless deposition is generally catalyzed by reduced metal sites on the surface to be coated. Such deposition is often initially catalyzed by reduced palladium compounds distributed over the surface. Once initiated electroless deposition is autocatalytic in that deposited reduced metal provides an expanding catalytic surface for further deposition.
Considerable effort in the art of electroless deposition has been devoted to improving the quality of electrolessly-deposited metal coatings. U.S. Pat. No. 3,414,427 discloses that better adhesion of metal coatings is achieved by use of a more soluble complex, e.g. of palladium chloride, hydrogen chloride and water. Other developments based on modified palladium complexes are disclosed in U.S. Pat. Nos. 3,520,723 (cuprous iodide treatment), No. 3,847,648 (ketopalladium complexes), No. 3,937,857 and No. 4,006,047 (thermodecomposable palladium complexes) and No. 3,963,841 (dimethyl sulfoxide complexes).
Other attempts to improve metal coating adhesion have included etching of polymeric substrates, e.g. with chromic and/or sulfuric acid. See, for instance, U.S. Pat. Nos. 3,370,974; 3,423,226; 3,437,507; 3,507,681; 3,515,649; 3,616,296; and 3,702,286 which disclose various acid etching techniques which are often useful in preparing surfaces comprising ABS (a multiphase thermoplastic of dispersed butadiene with grafted styrene acrylonitrile copolymer).
Another method of improving adhesion is disclosed in U.S. Pat. No. 3,488,166 where reducible catalytic salts are bonded to formaldehyde resin substrates.
The following list of treatments further exemplify a wide variety of techniques disclosed in U.S. Patents for improving the quality of electroless deposition: (a) substrates of aromatic polyamines are provided with hydroquinone radicals (No. 3,523,874), (b) polystyrene surfaces are treated with ethoxylates (No. 3,533,828), (c) substrates of carboxylic acid polymers are treated with ammonia or alkylenimines (No. 3,567,488 and No. 3,567,489, respectively), (d) PVC substrates are dehydrohalogenated and oxidized (No. 3,639,153), (e) polymeric surfaces are treated with quaternary amines (No. 3,684,572), (f) hydroxy functional filler is provided in polymer substrates (No. 3,701,675), (g) bipyridyls are incorporated into substrates (No. 3,853,589), (h) cyano-containing resins are incorporated into substrates (No. 4,017,265), (i) substrates are treated hydrosols of compounds having at least two oxygen atoms (No. 4,021,314); (j) substrates are pretreated with phosphorus compounds (No. 4,063,004), (k) substrates are subjected to glow discharge (No. 4,250,225), and (1) discrete crystalline/amorphous regions are developed in polyphenylene sulfide substrates (No. 4,486,463).
In still other cases, e.g. as disclosed in U.S. Pat. Nos. 3,347,724; 3,523,824 and 3,642,476, particles of reducible catalytic metal compounds have been adhered to a surface by incorporating such compounds into a binder, such as a thermoplastic resin. See also U.S. Pat. No. 3,560,257 where organic compounds of Group 1B and 8 metals are used with a variety of extenders (including polymers) to provide bonding between the substrate and the catalytic organic compound. See also U.S. Pat. No. 3,900,320 which discloses the use of thin polymer layers to provide a readily reducible catalytic metal salt on a substrate. Such layers are formed from polymer solutions where the weight ratio of polymer material to catalytic metal compound is required to be substantially less than about 15 to 1.
Because of the high cost of preferred catalytic metals, e.g. palladium, an objective has been to find ways to utilize lower levels of catalytic metal compound in combination with polymeric binders. In this regard, see U.S. Pat. No. 3,930,109 which discloses the application of thin films from dilute solutions or suspensions of polymeric binder and catalytic compounds; useful films are prepared from polymer solutions containing as little as 0.4 percent palladium chloride based on the weight of the polymer component. See also U.S. Pat. No. 4,493,861 which discloses the use of palladium complexes of (poly)butadiene from solutions with as low as 5.2 percent by weight of palladium (based on anhydrous polybutadiene). A disadvantage of such films is that the entire film surface is platable by electroless deposition. This means additional process steps are required when plating of less than the entire surface is desired, e.g. as in the production of printed circuitry, selective decoration or selective shielding.
In this regard selectivity in surface coating by electroless deposition has been provided in a variety of ways. For instance, U.S. Pat. No. 3,615,471 discloses methods of making optical masks by coating a transparent plate (e.g. glass) with a transparent photoresist layer comprising, for instance, polyvinyl alcohol and a chromate salt together with a plating catalyst. Such photoresist layer can be exposed to a light pattern and developed (e.g. solvent treated to remove the unexposed part of the layer) to leave a catalytic polymeric pattern which can be electrolessly coated.
Other methods involve the use of photosensitive materials, e.g. to promote reduction of the catalyst metal (see U.S. Pat. No. 3,772,056) or to otherwise generate catalytic nuclei (see U.S. Pat. Nos. 3,719,49; 3,779,785; 3,900,320; 3,925,578; 3,942,983; 3,959,547; 3,994,727; and 4,560,643.) See also U.S. Pat. No. 3,672,986 which discloses the production of printed circuitry by electrolessly depositing a metal coating over the entire surface of a board substrate. A patterned mask is applied to allow electroplating of the desired circuitry. The mask is removed to allow chemical etching of the undesired portions of the original electroless deposit. Other mask technigues are disclosed in U.S. Pat. No. 3,642,476. See also U.S. Pat. No. 3,656,952 which discloses films containing palladium salts and photosensitive compounds that allow electroless deposition of photographic images.
Other technigues for electroless deposition over selected surfaces, e.g. for printed circuitry, are disclosed in U.S. Pat. No. 4,368,281 where a precursor of the desired circuit is printed with a catalytic ink, e.g. a solution of resin, crosslinker, dye and high levels (5-15 by weight) of palladium complexes. See also U.S. Pat. No. 4,574,095 where vaporized palladium complexes are deposited as clusters on a laser-irradiated pattern of polymer surface.
Since electroless deposition techniques generally involve a multitude of steps or somewhat sophisticated procedure which, as indicated above, may include pretreatments, etchings, washings, masking, catalyst reduction and the like, there is still a need for simple, effective electroless deposition technology which can utilize low levels of expensive catalyst compounds with minimal waste.
In addition to the above-mentioned difficulties attendant with electroless deposition processes it has been generally found that electrolessly deposited coatings of oxidation susceptible metals, e.g. copper, tend to rapidly discolor and lose their metallic appearance and properties. Prevention of such oxidation is generally achieved by applying a protective coating, for instance, of electrolessly deposited or electroplated nickel, electroplated copper, thermoplastic polymer or thermosetting resin.